FIG. 1 is a block diagram of a conventional pulsed EMI metal detector and method of operation. A current loop transmitter 10 is placed in the vicinity of the buried metal target 12, and a steady current flows in the transmitter 10 for a sufficiently long time to allow turn-on transients in the soil (soil eddy currents) to dissipate. The transmitter loop current is then turned off. The transmitter current is typically a pulsed waveform. For example, a square-wave, triangle or saw-tooth pulsed waveform, or a combination of different positive and negative current ramps.
According to Faraday's Law, the collapsing magnetic field induces an electromotive force (EMF) in nearby conductors, such as the metal target 12. This EMF causes eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver 14 located above the ground and coupled to the transmitter 10 via a data acquisition and control system 16.
Pulse induction metal detector (PIMD) antennas (transmitter and receiver coil) come in two basic types as shown in FIGS. 2a and 2b. The first type of PIMD uses a single transmit and receiver coil 22 with multiple loops of wire forming the coil (FIG. 2a). A current pulse is sent through the multiple turn coil 22 and the received metal detection signal is sensed by the same coil 22. The small voltage generated by the metal target is typically amplified by a high gain electronic amplifier 25 (typical gain factor of 100 to 1000). A protection circuit is provided to protect the sensitive amplifier from the high kick-back voltage pulse generated by switching the inductive coil off abruptly (V=L di/dt, where L is the inductance of the transmitter coil and di/dt is the slope of the current decay in the coil). The second type of PIMD uses a separate transmitter coil 23 and receiver coil 24, again, with multiple loops of wire forming the coils (FIG. 2b). This configuration provides isolation between the transmitter circuit and the receiver circuit and allows for more flexibility in the receiver coil 24 (e.g., different number of turns, size or differential coil configuration) and amplifier circuit design (e.g., single ended operation of electronics). The high gain amplifier 25 also sees the high kick-back voltage pulse generated by switching the transmitter coil 23 off abruptly and protection circuitry is needed to protect it from damage.
Two basic problems exist with prior art PIMDs. First, the high kick-back voltage of the transmitter coil 23 temporally “blinds” the receiver coil 24 from amplifying metal target signals near the turn-off time of the transmitter coil 23. The transmitter coil 23 is an impulse excitation to the receiver coil 24, and as such, the receiver coil 24 will have a decay voltage proportional to the inductance of the receive coil 24. Receiver coils typically have many turns for increased sensitivity and therefore, have relatively large inductances. These large decay voltages can persist for many microseconds and mask the signal from very small metal targets. Second, the protection circuitry typically has a delay time that also temporally “blinds” the receiver coil 24 from amplifying metal target signals near the turn-off time of the transmitter coil 23. Some protection circuitry uses switches to disconnect the receiver coil 24 from the amplifier 25 during the period that the kick-back voltage would cause amplifier saturation or damage. Low noise, high gain, low bandwidth amplifiers take time to come out of saturation which makes them “blind” to metal target signals. Other protection circuitry uses diodes to limit the voltage to the amplifier 25.
FIG. 3 illustrates the concept of “receiver blindness” with real data for the PIMD configuration of FIG. 2a. The display scale is 10 μs per division. Trace (A) shows a transmitter current of 5 A switching off abruptly in less than about 10 μs. Trace (B) shows the output from a fast-recovery amplifier (gain of 200) with fast recovery protection diodes typical of a PIMD. The large peak of Trace (B) is a saturation peak caused when the transmitter current is switched off. As the figure shows, the receiver coil has a large decay voltage from the transmitter pulse and the amplifier comes out of saturation at about 36 μs after transmitter turn-off. To sense metal signals, the time is closer to about 45 μs. Thus, metal signals and decay signatures are obscured during the 36 μs that the amplifier is in saturation.
What is needed is a system and/or method to sense metal signals closer to the transmitter turn-off time for a given PIMD coil configuration.